Specific and selective degradation of protein by illumiation

光系统Ⅱ反应中心D1蛋白在强光照射过程中选择性、特异性降解。Q_A功能的丧失是导致光系统II光抑制功能丧失的主要原因。不同类型的光系统II抑制剂的存在都能抑制或刺激D1蛋白的光抑制作用，表明抑制剂结合引起的Q_B结合位点的变化调节了D1蛋白的降解。事实上，D1蛋白的切割是在光系统II抑制剂PNO 8（N-辛基-3-硝基-2，4，6-三羟基苯甲酰胺）存在下在完全黑暗下进行的。在其它PS Ⅱ抑制剂DCMU存在下，切割被抑制，表明在Q_B-位点的PNO_8的结合负责切割反应。在D1蛋白的去环区中Lue 258附近的一个位点进行切割，得到23和9 kDa片段，其也在通过照射强光的光抑制期间发现。在低温和丝氨酸蛋白酶抑制剂的作用下，酶的裂解受到抑制，但不受氧的影响。分裂不需要光，而是由短时间预照射触发。结果表明，光照和PNO 8的结合导致了QB结合位点的某些变化，从而使D1蛋白处于允许切割反应的状态。

A photosystem II reaction center D1 protein was slelctively and specifically degraded during the course of strong light illuminaition, photoimhibition. The loss of the function of Q_A was primarily responsible for the loss of the photosystem II function by photoinhibition. The photoinhibition was sppressed or stimulated by the presence of varios type photosytem II inhibitors, suggesting that the changes in the Q_B-binding site by the binding of the inhibitors modulates the degradation of the D1 protein. In fact, the cleavage of the D1 protein was prrformed under complete darkness in the presence of photosystem II inhibitor, PNO8 (N-octyl-3-nitro-2,4,6-trihydroxy benzamide). The cleavage was inhibited in the presence of other PS II inhibitor, DCMU,indicating that the binding of PNO8 in the Q_B-site is responsible for the cleavage reaction. The cleavage was performed at one site in the vicinity of Lue 258 in the de-loop region of the D1 protein to give 23 and 9 kDa fragments that are also found during photoinhibition by illuminating strong light. The cleavege was inhibiet under low temperature and inhibitors of serine type protease but was not affected by the presence of oxygen. The cleavage did not require light but was triggered by short time pre-illumination. The results suggest that some changes in the Q_B-binding site by illumination and the binding of PNO8 induce the state which allows the cleavage reaction of the D1 protein.